Satellite antenna systems are often used to provide communications between mobile ground-based terminals. Reliable communications between terminals is preferable to all users however, some applications have an especially critical need for robust operation. Military applications, for example, require a system that maintains communication between highly mobile terminals even in the presence of jamming signals and other sources of interference. Although jamming will be discussed below, the invention is equally applicable to any source of interference in a communication signal, intentional or unintentional.
Beamforming is a technique wherein beams from a plurality of transmitters and/or receivers are combined to provide directional signal transmission or reception. Beamformers are especially useful in the presence of a jamming signals, where a null can be used to cancel out the jamming signal while the antenna is still able to listen to signals from other directions. In a communication environment with large jamming sources in the same receive bandwidth as the preferred directional signal from a terminal, communication performance will be degraded significantly if anti-jamming measures are not employed. Numerous techniques have been proposed for dealing with this type of problem. Nulling approaches (Howells-Applebaum, U.S. Pat. No. 5,175,558, and U.S. Pat. No. 6,130,643) all attempt to preserve an overall pattern performance to all possible users, across the full system bandwidth, in the presence of jammer signals. An antenna with a nuller uses beamforming to constructively add signals from a desired source such as a mobile terminal, and cancel out the signals from a jamming or other undesired source. These approaches, however, are suboptimal.
One example of a prior art system using analog beamforming is shown in FIGS. 1A-1C. FIG. 1A depicts a system in a benign environment, where three pixel beams 202 are combined into a single composite theater beam 204. Three mobile terminals are shown in the theater at 206. Although three pixel beams are shown, any number could be used. FIG. 1B depicts a similar system of three pixel beams 202 in a contested environment having three mobile terminals 206. Composite theater beam 208 is distorted by the presence of jammer 210. While FIGS. 1A and 1B show three mobile terminals, this is just representative and any number could be present.
An apparatus for generating composite beams 204 and 208 is shown in FIG. 1C. A multiple beam antenna (MBA) 212 is shown with 7 beams, although any appropriate number of beams could be used. The output from MBA 212 is sent to analog beamformer 214 which generates composite beam 204 or 208 of FIGS. 1A and 1B respectively. Analog to digital converter (ADC) 216, channelizer 218 and demodulator 220 represent the payload architecture that processes the received signal digitally and demodulates user symbols in baseband. A negative feature of this apparatus is the requirement for a large and complex antenna. In addition, the traditional beamforming performed in the prior art requires a known scan direction and a known antenna aperture model to steer the beam in the desired direction.
In addition, communication with terminals in a theater of operations may experience several challenges. First, it is necessary to provide high gain to small power terminals. The theater may feature a heterogeneous environment with large and small power terminals close together spacially and in frequency. Finally, it may be necessary to provide high anti-jamming capabilities in a contested environment and also to enable autonomous tracking of small power terminals in the theater.
Approaches using a diverse set of antenna elements or an unknown aperture suffer from antenna model inaccuracies that severly limit performance and also experience grating lobes that contribute additional interference. Thus, there exists a need for a satellite antenna system that can perform adaptive beamforming without knowledge of signal direction or aperture. Furthermore, there is a need for a method that optimizes for each user independently only over the user receive bandwidth.